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64 DESIGN007 MAGAZINE I SEPTEMBER 2021 cators, as we all learn how to best apply and take advantage of these new capabilities. As I diligently work to disseminate informa- tion and to help shorten this learning curve for both designers and fabricators, I am routinely asked for "design rules" for SAP and mSAP. at is a fair request; aer all it is what we have been requesting from fabricators for decades. Only this time, it is a little trickier. ere is tech- nology capability involved with creating a PCB trace and space with semi-additive or modified semi-additive processes that is dependent on each fabricator's equipment. ese processes change the manufacturing constraint for trace formation from the etching process to the pho- tolithography process; feature size capabilities will differ depending on equipment capabili- ties and photoresist. In fact, SAP technology can go far below the 25-micron trace and space that we see in marketing materials and if your fabricator has photolithography equipment that goes to 5 microns, they can produce at that feature size. Semi-additive and modified semi-additive processes also have very little direct impact on other PCB fabrication processing steps that come before or aer the trace and space forma- tion processes. Drilling is a good example. If your fabricator can only offer a 6-mil mechani- cal drill with subtractive etch processing, they can still only offer that feature size aer install- ing a semi-additive process, unless they also make a capital investment in drill equipment at the same time. Materials is another good example. If your fabricator is currently running flexible materials, rigid-flex, and exotic materi- als, these are all compatible with semi-additive processes, but as we have all experienced, not every fabricator specializes in these materials. at is independent of the circuit formation, semi-additive processes. I think that is fairly intuitive. Here is where things get a little trickier and we start enter- ing the territory of moving through a learn- ing curve. Semi-additive PCB fabrication techniques don't require every layer to be done with semi-additive technology; in fact, it is common to mix layers of subtractive etch with semi-additive layers. One thing to con- sider is what we are terming "layer pairs." As an example, if layer two is using semi-additive processes, the related layer on that core should also use semi-additive processes. It is not strictly required, but the fabrication process will be much simpler, resulting in improved yield and reduced cost. is is also related to fabricator capabilities and preferences—rein- forcing the need for designers and fabricators to work collaboratively with this new technol- ogy. Let's look at another example. e starting point is an eight-layer design with five signal layers. As a starting point, we took the manu- facturing capability to 35 microns, a very com- fortable feature size for semi-additive process- ing. is resulted in a reduction to a four-layer design with two signal layers and two power ground layers. Sounds impressive, doesn't it? It is and would certainly be a win for design simplification and manufacturability. But, working collaboratively, we realized that the impedance control in that particular stackup would be impacted with traces with that tight pitch. Aer further review, it was decided to use 50-micron traces and 22.5 micron spac- ing to meet the impedance requirements. e As I diligently work to disseminate information and to help shorten this learning curve for both designers and fabricators, I am routinely asked for "design rules" for SAP and mSAP.

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